Titanium alloys are preferred materials for the construction of propulsion plants, such as aircraft engines and the like, due to the advantageous strength to weight ratio, especially with regard to the vibratory material strength and the weight of the titanium alloys. The operational temperature for such alloys, however, is limited because at temperatures exceeding 550.degree. C. a pronounced oxidation and corrosion set in. So-called titanium fires may be caused by local overheating, whereby the material combusts and discharges glowing titanium particles which may partially even be in the form of a flowing melt. Such titanium fires also have the tendency to spread.
German Patent Publication (DE) 3,742,944 discloses a method for producing a coating for protecting against oxidation, by diffusion of aluminum and niobium into the surface of the titanium for protecting the titanium against oxidation. Simultaneously, the diffusion forms intermetallic phases which harden the material surfaces so that the wear and tear characteristics are improved. Incidentally, German Patent Publication 3,742,944 corresponds to U.S. Ser. No. 07/283,745, filed on Dec. 13, 1988, now U.S. Pat. No. 4,935,193. Although the diffusion coating or the aluminizing of the titanium structural components results in an effective oxidation prevention, there is room for improvement with regard to some brittleness that is caused by a surface hardening due to said intermetallic phases. Such brittleness could reduce the vibratory material strength up to 30% compared to the same material without such intermetallic surface hardening phases.
Further, the oxidation preventing coating according to the just mentioned German Patent Publication 3,742,944 is not intended to provide protection against titanium fires, nor against the spreading of titanium fires by means of aluminized protective coatings. Similar considerations apply to wear and tear resistant protective coatings according to German Patent Publication 3,321,231 in which nickel is diffused into the base material of titanium, whereby the nickel forms intermetallic phases with the titanium and these intermetallic phases are rather wear resistant, however, they are also brittle. Such brittleness is undesirable. Further, platinum coatings or layers on structural components made of titanium are also known for corrosion protection purposes. However, such platinum coatings also do not provide an effective protection against titanium fires.
The above mentioned conventional protective coatings are very thin, the thickness is generally less than 2 .mu.m and these conventional layers are hard. As a result, there is the danger that even a minimal mechanical influence removes the protective coating, thereby exposing the base material in small micro-surface areas. If such bared surface areas are exposed to localized overheating as may occur, for example, in compressors of gas turbines, a titanium fire may be caused and the resulting droplets of titanium melt are hurled through the compressor due to the high flow speed. The travelling titanium melt droplets may damage other titanium surfaces downstream of the initial titanium fire so that damaged structural components may burn altogether which may lead to the shut-down of the entire propulsion plant.